The present invention generally relates to a method for the thermal treatment of a substrate, such as a wafer; more particularly, the invention relates to a method comprising the introduction thereof of a substrate in a heat treatment apparatus, wherein the heat treatment apparatus comprises two substantially flat parts parallel to the introduction position of the wafer, between which the wafer is received, wherein the first part is heated and is at a first high temperature and on each of the two sides of the wafer a gas flow is supplied to implement the heat treatment.
A method for treating a substrate is disclosed in WO 98/01890 of ASM International. It appears that particularly quick warming up is possible when introducing a wafer in such a heat treatment apparatus (floating wafer reactor), as a strong thermal coupling between the wafer and reactor parts can be brought about. For various processes it is important that heating up takes place quickly so that the transition period in which certain reactions continue in an uncontrolled manner is kept as short as possible.
Problems nevertheless occur at the end of a reaction carried out at an elevated temperature. In practice, it has shown to be particularly difficult to cool down wafers in a controlled manner. It is clearly possible to take these out of the reactor and to let them cool down in the air. While such a cooling is reasonably quick, it has been shown that it can cause tensions in the wafer, as the outer edge cools off more quickly than the middle of the wafer. To avoid this problem, it is proposed to position a ring with considerable thermal capacity around the wafer. In this way, an even cooling is realized, but a quick decrease in temperature cannot be realized. It can therefore not be guaranteed that the reaction during various sorts of thermal/chemical treatments can be stopped in a controlled manner after a certain treatment time.
Some embodiments of the present invention seek to avoid these disadvantages and to provide a more accurate controller wherein it is possible to quickly cool down and likewise to heat up, wherein the gradient is manageable in a controlled manner. This is achieved in the method described above in that, during said treatment, the second part is cooled with the help of cooling means and is at a temperature lower than 70xc2x0 C. and in that, during the treatment, the heat conductance between the wafer and at least one of those parts is controlled in such a way that, during a certain time, the wafer takes on a temperature that is comparatively closer to the first, high temperature and then takes on a temperature which is comparatively closer to the second lower temperature.
It has been found that in a floating wafer reactor and also other constructions wherein the wafer is heated via heat conduction through a gas, a particularly quick heating is possible as the reactor parts can be brought to a very small distance away from the surface of the wafer whereby a strong thermal coupling between the reactor part and wafer occurs. This distance is preferably less than 1 mm and can in particular be less than 0.1 mm. As the heat capacity of the reactor parts is several times larger than the heat capacity of the wafer, when the two reactor parts are at the same temperature, the wafer will take on the temperature of the reactor parts particularly quickly while the temperature of the reactor parts will not noticeably change as a result.
The configuration of the supplied gas flows can be an important factor for achieving this effect. For example, if gas is flowed against a wafer from two sides, when the gases have different heat conduction and the wafer is in the middle between the two parts from which the gas flows, the wafer will be more strongly thermally coupled to the reactor part from which the better conducting gas flows. In a floating wafer reactor, it is comparatively easy to give the opposing parts different temperatures. The wafer will then take the temperature which lies between the temperatures of the two reactor parts. In floating wafer reactors, gas is usually lead through channels in the opposing parts which bound the treatment chamber. Due to the heat capacity of the surrounding material, it is not easy in practice to quickly change the temperature of the gas that flows from a certain part in a controlled manner. However, it is possible to change the gas itself in an easy manner. That is, if in the first situation a good conducting gas is used on one side with low temperature and on the other side with high temperature a less well conducting gas is used, then the substrate will take on a steady temperature which is comparatively closer to the low temperature. If the temperature of the reactor parts is kept the same and the gasses are swapped, that is, that a less well conducting gas is brought into the treatment area via the reactor part which is at a low temperature, and a good conducting gas is brought into the treatment area via the reactor part which is at a higher temperature, then the wafer will heat up particularly quickly. The only change with respect to the original situation is the switching over of the gas type. This same treatment, but then in the opposite direction, can be used starting from that raised temperature, to cool the wafer down particularly quickly.
It has been found that using the embodiment described above a particularly steep temperature gradient can be achieved. It will also be understood that many types of treatment at raised temperature are suitable for this method. This temperature used depends on the treatment and is generally under 400xc2x0 C. Every gas which meets the desired requirements can be used for this. It is known that the heat conductance of nitrogen is substantially lower than the heat conduction of helium so that, by combining these two inert gasses, quick heating up and then quick cooling down can take place.
The quickest change in temperature of the wafer will be achieved when the switching over of the gas type is carried out abruptly, by instantaneous switching of the gas flows. However, the gases can also be switched more gradually so that the switching occupies a certain period of time. In this way, the temperature ramp rate to which the wafer is subjected during switching can be controlled.
In some embodiments, it is possible to use gasses which in some way cause or influence a chemical reaction. In another embodiment of the invention in which the wafer is kept floating in place by the gas flows coming from the two parts of the reactor, it is possible, instead of or in combination with the method described above, to change the distance of the wafer to the two opposing parts of the floating wafer reactor by controlling the size of the gas flow. It will be understood that if the wafer is very close to one part and at a larger distance from the other part, the effect of this positioning can be compared to the use of a more or less well conducting gas.
Depending on the differences in conduction or other heat capacity, the wafer will take on a temperature which lies somewhere between the temperature of the two spaced parts.
The method described above is very flexible and can be implemented in many types treatments. For example, in copper annealing, a copper layer is annealed after deposition at a temperature of 250xc2x0 C., for example, to achieve a low resistance of the copper. Annealing times in the range of 1-90 seconds are usual here. Considering the short treatment time, forced cooling of the wafer is desirable to realize sufficient production capacity. It is important to keep the wafer in an inert atmosphere when the wafer temperature is 100xc2x0 C. or more with the intention of keeping the oxidation of the copper within acceptable limits. For this reason, when a separate cooling station is used, the transport of the wafer from the anneal station to the cooling station should be carried out in an inert atmosphere. In a method according to one embodiment of the invention, a significant simplification and reduction of the apparatus takes place because heating up and cooling down take place in the same reactor chamber and transport of the wafer between times is not necessary. The annealing of low-k dielectric materials at a temperature of 200xc2x0 C. to approximately 400xc2x0 C. can be given as another example.
Some embodiments of the invention relate to a heat treatment apparatus comprising two substantially flat parts opposite each other and parallel to the wafer, between which the wafer is received, wherein the first part is provided with heating means to bring this first part to a first high temperature and wherein each of said parts is provided with gas supply channels which open in the area between both parts, characterized in that the second part is provided with cooling means to keep this second part at a second low temperature wherein this temperature is lower than 70xc2x0 C.